The classical way of transmitting the contents of orthogonally scannable data arrays such as television images is to sample them line by line with pulse-code modulation (PCM) and reorganize the serially arriving PCM samples into a recurrent frame at the receiving end. Numerous attempts have been made to lower the bit rate required for such transmission, in order not to exceed the maximum operating rate of the channel employed, without sacrificing significant details in the visual representation. Various transcoding techniques designed to reduce the number of bits include linear transformations of the Fourier, Hadamard and other types described, for example, in an article titled Hadamard Transform Image Coding by William K. Pratt, Julius Kane and Harry C. Andrews, Proceedings of the IEEE, Vol. 57, No. 1, January 1969, pages 58-65, and an article titled Image Coding by Linear Transformation and Block Quantization by Ali Habibi and Paul A. Wintz, IEEE Transactions on Communication Technology, Vol. Com-19 , No. 1, February 1971, pages 50-63.
By subdividing each array or frame into a multiplicity of elemental data blocks, generally of rectangular shape, it is possible to allocate different numbers of bits to the several blocks in accordance with their relative degrees of activity in terms of signal energy. Reference in this connection may be made to such publications as an article titled Use of "Activity" Classes in Adaptive Transform Image Coding by James I. Gimlett, IEEE Transactions on Communications, July 1975, pages 785 and 786; an article titled Adaptive Coding of Monochrome and Color Images by Wen-Hsium Chen and C. Harrison Smith, IEEE Transactions on Communications, Vol. Com.-25, No. 11, November 1977, pages 1285-1292; and an article titled "Adaptive Transform Image Coding for Human Analysis by O. R. Mitchell and Ali Tabatabai, ICC 1979, Boston, pages 23.2.1-23.2.5. Reference may further be made to an article by S. Brofferio et al in the Italian magazine "Electronica e Telecommunicazioni", Vol. No. 5, 1978, pages 195-208.
According to commonly owned U.S. patent application Ser. No. 515,362, filed 19 July 1983 by Leonardo Chiariglione and Mario Guglielmo, a two-dimensional data array such as an orthogonally scanned television image is divided into a multiplicity of elemental blocks each containing a given number of PCM samples which are converted by linear transformation into a series of coefficients to be quantized for transmission to a remote receiver. The data blocks are processed one at a time by an iterative coder which allots a progressively increasing number of bits to a given block in successive operating stages, the bits being allocated to the several coefficients of the block according to the position of the barycenter of their signal energy assigning the block to one of several classes. An initial overall reconstruction error, computed from the coefficients of a block with no bits yet available for quantization, is updated with each bit allotment in the course of an iteration in which a selected coefficient is subjected to simultaneous coding with a number of quantizers differing from one another in their quantum steps. The coding operations of the concurrently active quantizers have different error-reducing effects and the resulting minimum error is compared with a threshold which itself may be variable according to the activity of the image block being processed. When the residual overall error falls below that threshold, processing is terminated and the bits used in the coding of each coefficient are sequentially sent out together with a count of the total number of bits allotted to the block and a parameter indicating its classification, this information enabling the receiver to reconstruct the image within the limits of the tolerated residual error.
Another known redundancy-reducing method calls for comparing all picture elements or image points on a scanning line of a current image frame, referred to in the art as pixels, with homologously positioned pixels of the immediately preceding frame and generating a reduction code for any number of consecutive pixels which are found to have substantially the same luminance as their homologs in the preceding frame. Digital signals so coded at the transmitting station are then sent to the receiving station for decoding by a reverse procedure so as substantially to reconstruct the original message signals.
The coders and decorders used at the two stations generally comprise microprogrammed or wired logic circuits particularly designed for the purpose at hand. Since the encoded data words constitute messages of variable length, depending for example on the number of consecutive pixels whose luminance values are not being sent out inasmuch as they substantially equal those of respective earlier homologs, the outgoing bits must also be stored for varying times at the transmitting station in order to be sent at regular intervals over the channel. Preliminary storage is also required at the receiving station in order to provide time for the re-expansion of the incoming data words and for integration of the reconstructed message signals into a uniform data flow used, for example, to control the beam of a monitoring video tube.
The re-expansion of the received information may also lead to the amplification of transmission errors introduced by the digital channel. These errors may require a "stuffing" of the resulting data flow, according to an expedient widely practiced in the reproduction of video images, with signals replicating a preceding frame so as to hold the displayed image stationary for a limited period.